pgsql: Just noticed that you can't Query-Cancel a long planner run,

Log Message:
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Just noticed that you can't Query-Cancel a long planner run, because
no part of the planner did CHECK_FOR_INTERRUPTS(). Add one in a
suitably strategic spot.

Modified Files:
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pgsql/src/backend/optimizer/util:
pathnode.c (r1.120 -> r1.121)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/pathnode.c.diff?r1=1.120&r2=1.121)